nextFrame playhead stops all Movieclips?

I’m learning to code
http://torbach78.carbonmade.com/projects/3225767

its the same structures we see in many online tutorials obviously, what is bugging me is during up and down i have the ship perform some roll action (rtype knock off)this.nextFrame & this.prevFrame seem to cause the playheads to stop in the engine/thrust movieclip; even though its timeline, (i thought), was independent.

my solution was make ourBody inside ourShip…call next/prev frames to just ourBody and leave the engine+thrust animation movieclips out of that process…
dur…

i can see advantages to having control over the engine/body separately but feels like a bit of a band-aid, should I be thinking more in terms of flashes default art ability to have a movieclip play regardless of parent timelines as the “unusual” logic here?